1. Field of the Invention
This invention is in the field of integrated circuits, and is more specifically directed to the fabrication of embedded flash.
2. Description of the Prior Art
Non-volatile memory devices are important for they provide an advantage that random access memory (RAM), both dynamic and static, cannot provide. That is, non-volatile memory devices do not lose their memory even the power is turned off.
Read only memory (ROM) is the most popular variety of nonvolatile memory devices. ROMs come in a number of visions and a relatively advanced type of ROMs is the flash memory that uses a technology similar to the erasable programmable ROM.
However, the flash memory is electrically reprogrammable for a limited number of times. This makes it ideal for those applications where only a few changes in the programming of the system is for either the entire memory array or for blocks of it.
Beside, memory storage exists not only as stand-alone memory device, but also embedded in processor chips. The performance of an embedded flash can be better than other types of flash since bandwidth problems are reduced and interface circuit and package leads are eliminated. It can also have characteristic tailored to the specific application rather than being a standardized comprise between many factors such as high operating speed.
The most obvious limiting factor for an embedded flash is the cost. Another serious disadvantage of the embedded flash is relevant fabrication. Owing to the fact that in conventional fabrication both spacers and silicides of transistors of both cell (memory) region and (peripheral) logic region are formed simultaneously, conventional embedded flash has the following main disadvantages:
(1) Spacers of transistors of both peripheral region and cell region are formed by the same material. Therefore, quality of transistors of both peripheral region and cell region cannot be optimized at the same time. In other words, either performance of any transistor of a peripheral cell is degraded or reliability of any cell region is degraded.
(2) Sources/drains of transistors of cell regions are covered by silicides, therefore, junction breakdown voltage is decreased and junction leakage is increased and qualities of memory cells are degraded.
(3) The ultra-violet (UV) erasing process of the cell region is degraded by the borderless contact stop layer.
Therefore, it is indisputable that development of a new fabrication of the embedded flash to overcome these disadvantages of conventional fabrication is desired. And it is more important when the flash is an irreplaceable product of ultra large scale integration (ULSI). @